The term “bulk-solidifying amorphous alloys” refers to a family of amorphous alloys that may be cooled at rates of about 500 K/sec or less from their molten state to form objects having thicknesses of 1.0 mm or more while maintaining a substantially amorphous atomic structure. Bulk-solidifying amorphous alloys' ability to form objects having thicknesses of 1.0 mm or greater is a substantial improvement on conventional amorphous alloys, which are typically limited to articles having thicknesses of 0.020 mm, and which require cooling rates of 105 K/sec or more. Bulk-solidifying amorphous alloys, when properly formed from the molten state at sufficiently fast cooling rates, have high elastic limit typically in the range of from 1.8% to 2.2%. Further, these amorphous alloys may show bending ductility ranging from a few percent in samples of 0.5 mm thick or more to as high as 100% as in the case of 0.02 mm thick melt spun ribbons.
Generally speaking, bulk-solidifying amorphous alloy compositions have been found around highly deep eutectics. Highly deep eutectics is generally characterized and quantified by a reduced glass transition temperature, Trg, and is defined by the ratio of glass transition temperature to the melting temperature (in units of Kelvin). Herein, the melting temperature is generally understood as associated to the eutectic temperature. Generally, a high Trg has been desired to obtain easier bulk-solidification of the amorphous alloys. This relationship has been generally supported by both the classical theory of nucleation and experimental observation as well. For example, a Trg of 0.6 is observed for critical cooling rates of 500 C/sec, and a Trg of 0.65 or more is observed for critical cooling rates of 10 C/sec or less.
U.S. Pat. Nos. 5,032,196; 5,288,344; 5,368,659; 5,618,359; and 5,735,975 (each of whose disclosures is incorporated by reference in its entirety) disclose such families of bulk solidifying amorphous alloys. In addition, cast articles of these alloys in the form of in-situ composites have also been disclosed.
The discovery of bulk-solidifying amorphous alloys and the discovery that these alloys can be cast into articles having substantial thicknesses allows for the possibility of incorporating these high elastic limit materials in bulk form for a wide variety of applications. As such, a practical and cost-effective method to produce articles of these alloys is desired, and particularly for those applications that require designs of intricate and precision shapes. It has been found that metal mold casting methods, such as high-pressure die-casting, can be used to cast these materials as these methods provide high cooling rates. For example, U.S. Pat. Nos. 5,213,148; 5,279,349; 5,711,363; 6,021,840; 6,044,893; and 6,258,183 (each of whose disclosures is incorporated by reference in its entirety) disclose methods to cast articles of amorphous alloys.
However, it has been discovered that the presence of incidental impurities, such as oxygen, (when they exist in the alloy above certain concentrations) can detrimentally increase the rate of nucleation of crystals from the under-cooled melts of the bulk-solidifying amorphous alloys and accordingly increase the critical cooling rates of these materials substantially. For example, U.S. Pat. No. 5,797,443 discloses as a result of the presence of impurities, these alloys cannot be cast into the desired thick sections, and further teaches the necessity to control the level of oxygen impurities when casting bulk-solidifying amorphous alloys. One proposed method to control incidental impurities, such as oxygen, is to use higher purity raw materials and much more strictly control processing conditions. However, these steps substantially increase the cost of articles made of bulk-solidifying amorphous alloys.
Accordingly, a need exists for new bulk-solidifying amorphous alloy compositions and new methods to cast these alloys into articles inexpensively without the concerns raised by incidental impurities arising from both raw materials and processing environment.